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maxillae and the premaxillae above, is a sine qua non for
mammalian comparison. In the more basal Theromorpha the
teeth are not so limited in position. Finally, to complete the
remarkable mammalian resemblance of the teeth of these reptiles,
it must be mentioned that in Tritylodon and Diademodon the
ropts of the molars, as we may fairly term them, though not
actually divided after the mammalian fashion, were deeply
marked by a groove, which suggests an incipient division or a
fusion of two distinct roots. Some of these facts of structure
may now be considered in further detail As to the incisors
and canines, it is sufficient to say that the numbers of the former,
and the shape of the latter, are in perfect consonance with a
derivation, of the Mammalia from this group. The molar series
can be divided into premolars and molars, at least in so far as
regards their shape ; for the anterior teeth are often smaller and
less complicated than those which follow, as is often the case with
the two series in mammals. The molar series also consist of teeth
in close apposition to each other and separated from the canines
by a diastema, which is a character of mammalian teeth. The
fact that in the reptile Cynognatlius and the mammal Jlyr
mecobius there are nine of these molar teeth in each half of each
jaw is perhaps not a point upon which it is desirable to dwell
with too much weight ; but the general fact that the molars are
further reduced in some genera of Theriodontia than in that
which has been mentioned, is clearly a matter of significance
when the ancestry of the mammals is under consideration.
The most interesting fact about the molar series in the
Theriodontia is that we meet with the two types of molars that
occur in the mammals. Cynogncttfaus and other genera have
molars which consist of a main cusp, and of one cusp before and
one after the main cusp ; in fact these teeth are triconodont as
in certain early mammals, a state of affairs which is believed
by the " trituberculists " (see p. 56) to have preceded the
tritubercular tooth. There are also " imiltitubercular " teeth,
especially well developed in Tritylodon, where they exactly
resemble those of certain Multituberculata, and whose structure
originally led to the placing of Tritylodon among the mammals of
that group. If there is any question about the mammalian nature
of this fossil, there remain several other Theriodontia in which
the multituberculism is well marked. It is so in Trirhacfoodon.